Recently, development research has been actively performed on white light emitting devices utilizing a blue light emitting element as a light emitting source. In particular, the white light emitting diodes using a blue light emitting diode element are light in weight, use no mercury, and are long in lifetime, and thus the rapid expansion of the demand can be expected in the future. It is to be noted that a light emitting device using a light emitting diode element as a light emitting element is referred to as a “light emitting diode.” A method, which is the most commonly adopted, for converting blue light from a blue light emitting diode element to white light is a method in which yellow, which is a complementary color of blue, is mixed with blue light from the blue light emitting diode element to obtain pseudo white. As described in, for example, Patent Literature 1, a coating layer including a fluorescent substance, which absorbs a part of blue light to emit yellow light, is provided on the whole surface of a diode element, which emits blue light, and a mold layer, which mixes blue light from the light source with the yellow light from the fluorescent substance, is provided ahead, whereby a white light emitting diode can be configured. As the fluorescent substance, YAG (Y3Al5O12) powder activated by cerium (hereinafter referred to as “YAG:Ce”) or the like is used.
In the structure of the white light emitting diode typified by the device disclosed in Patent Literature 1, which are commonly used now, however, the fluorescent substance powder is mixed with a resin such as epoxy and coated, and thus it is difficult to ensure the homogeneity in the mixed state of the fluorescent substance powder and the resin and to control the stabilization of the thickness of a coating film, and the like, and it is pointed out that color unevenness or variance of the white light emitting diode easily occurs. In addition, the resin to be mixed with the fluorescent substance powder has an inferior heat resistance to those of metals and ceramics, and thus the transmittance is easily reduced due to the deterioration caused by heat from the light emitting element. Accordingly, this is a bottleneck for increasing the output of the white light emitting diode, which is required now.
The present inventors have proposed from before a ceramic composite for light conversion including a solidified body in which multiple oxide phases containing a YAG:Ce fluorescent substance phase and an Al2O3 phase are continuously and three-dimensionally intertwined with one another, and a white light emitting device configured with a blue light emitting element and the ceramic composite for light conversion (Patent Literature 2). In the ceramic. composite for light conversion described above, the YAG:Ce fluorescent substance phase is distributed homogeneously, and thus homogeneous yellow fluorescence can be stably obtained, and the heat resistance is excellent because of being a ceramic. In addition, because it itself is a bulk body, it is not necessary to use a resin for configuring the white light emitting device, unlike the device disclosed in Patent Literature 1. The white light emitting device, thus, has a small color unevenness or variance, and is extremely preferable for increasing the output.
In a white light emitting device using a blue light emitting diode element and a YAG:Ce fluorescent substance, light from blue light emitting diode element, which is commonly used now, has a peak wavelength around 460 nm for blue (for example, CIE 1391 chromaticity coordinates (hereinafter referred to as “chromaticity coordinates”) Cx=0.135, Cy=0.08). This is because the luminous efficiency of the YAG:Ce fluorescent substance is increased in this wavelength area. Incidentally, the color of a YAG:Ce fluorescent substance whose emission wavelength is not adjusted (hereinafter referred to as “unadjusted YAG:Ce”) is yellow having a peak wavelength around 530 to 545 nm (for example, chromaticity coordinates Cx=0.41, Cy=0.56).
As for the white light emitting diode, the chromaticity range (color temperature) to be required varies depending on uses such as a display, a lighting or a backlight source, and thus it is necessary to select the fluorescent substance used according to the use. In order to stabilize the chromaticity of LED, it is more desirable to use one kind of fluorescent substance than simultaneous use of multiple fluorescent substances. It is essential, accordingly, to set the fluorescence dominant wavelength within a desired range as a standard of an emission wavelength in the YAG:Ce fluorescent substance having a broad fluorescence spectrum. In usual, the emission wavelength is adjusted by moving a peak wavelength of a fluorescent substance material to a long wavelength side or a short wavelength side.
As for a YAG:Ce fluorescent substance, it is known as prior art that the increased or decreased content of Ce as an activator can shift the peak of the fluorescence wavelength (Non-Patent Literature 1). Thus, the peak of the fluorescence wavelength of the YAG:Ce fluorescent substance can be moved by around 10 nm.
In addition, as for a YAG:Ce fluorescent substance, it is known as prior art that, for example, partial substitution of the Y element by a Gd element can shift the peak of the fluorescence wavelength to the longer wavelength side (Non-Patent Literatures 2 and 3). Patent Literature 2 proposes that a YAG:Ce fluorescent substance having a fluorescence wavelength thus adjusted to the longer wavelength side is combined with a blue light emitting diode element to configure a white light emitting diode, thereby obtaining white (chromaticity coordinates Cx=0.33, Cy=0.33).
In general, however, it is known that when the known substituent element is used in order to adjust the wavelength of the YAG:Ce fluorescent substance, the fluorescence intensity is usually reduced to less than half (Non-Patent Literatures 4 and 5).
In the ceramic composite for light conversion as described in Patent Literature 2, the present inventors also demonstrate that the adjustment of the composition of the entire solidified body can adjust the peak of the fluorescence wavelength in the range of 550 to 560 nm or 540 to 580 nm (Patent Literature 3).